What do the lines along the centre of the road mean?

Explanation: White lines painted on the road give you information about where you may drive and when.
A line that lengthens, with shortened gaps, means there's a hazard ahead. For example, it may mean there's a sharp bend coming up.
If you need to overtake another vehicle or turn, you must make sure that the road ahead is clear before you cross the lines.

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References: The Official DVSA Guide to Driving: Section 6, Highway Code: Rule 127, Know Your Traffic Signs: Page 62